Add ChildRef::is_child

[lxfind]

It's a common need to check whether a child_ref points to a specific child object.
Add is_child for this purpose. It also enforces the type parameter on ChildRef is the same as the type of the child object, which helps avoid mistakes of mixing up the types (as we have seen in #777).
However, there are also cases (still questionable) we need to check the id against a naked id instead of child object. In this case, we also introduce the unsafe version, but only expose it to a limited number of friend classes in the framework.
If in the future we see this a common need, we could open it up.